1. Field of the Invention
The present invention relates to a hot-dip galvanized steel sheet having a zinc-aluminum alloy galvanized layer (hereinafter referred to as the Zn-Al galvanized steel sheet), and a method for producing the. same. More particularly, the present invention relates to a method for preventing an intergranular corrosion of a galvanized layer and propagation of cracks due to intergranular corrosion which occurs when the Zn-Al galvanized steel sheet is stored for a long period of time inside a house or in a high temperature- and a high humidity-atmosphere (90.degree. C. or higher and 90% or higher of RH), and also for preventing an embrittled galvanized layer from peeling from the steel base, due to embrittlement of galvanized layer.
2. Description of the Related Arts
Zinc-galvanized steel sheet is the most widely used among surface-treated steel sheets. The requests by users for the qualities of zinc-galvanized steel sheets to be enhanced increase year by year. Recently, in order to provide products which can meet the demands for enhanced corrosion-resistance, workability, and paintability, sheets galvanized with multi-components, such as Zn-Al, have been seriously researched and developed.
When a zinc-galvanized steel sheet using an inexpensive base metal of zinc undergoes a secular change when inside a house or is exposed to a high temperature- and high humidity environment, intergranular corrosion occurs. When the intergranular corrosion advances, the galvanized layer becomes embrittled and, hence, peels away from the steel base. This phenomena of intergranular corrosion, embrittlement and peeling of the galvanized layer occurs frequently even in the case of a Zn-Al galvanized layer having an improved corrosion resistance, with the result that the quality is greatly impaired.
According to an example for improving the resistance against the secular peeling of a Zn-Al galvanized steel sheet disclosed in Japanese Unexamined Patent Publication No. 57-67153 by the present assignee, the composition of a galvanizing bath is adjusted to from 0.1 to less 0.2% of Al, from 0.1 to 0.5% of Sb, and from 0.01 to 5% of at least one element selected from the group consisting of Mg, Cu, Cr, Ni, Co and Mn, and 0.02% at the highest of unavoidable impurities, such as Pb, Sn, and Cd, the balance being Zn.
According to another example disclosed in Japanese Unexamined Patent Publication No. 58-177450, which is related to a method for producing a composite hot-dip galvanized steel sheet, the composition of a hot-dip Zn-Al galvanizing bath which contains from more than 40% to 70% of Al, and from more than 0.5% to 10% of Si, additionally contains at least one of 0.01 to 1.0% of Mg, 0.01 to 0.5% of Mn, and 0.01 to 2.0% of misch metal, as well as from 0.01 to 0.5% of Sb, and the contents of unavoidable impurities are controlled to 0.1% or less of Pb and 0.02% or less of Sn.
The above-mentioned methods intend to prevent the embrittlement of a galvanized layer while not impairing an inherently high corrosion resistance of Al of the Zn-Al galvanized layer.